Has anyone done this, where you grow algae on rocks in a bucket, and then drop a rock in the tank for meals?

I put a bucket filled with old tank water and a drop of Flourish out in the sun with rocks yesterday. I checked it today, and there's brown fuzzy stuff in the water, the color of diatoms. I have diatoms on my java fern in my tank which they haven't touched, even though I thought they were supposed to love it. Am I growing diatoms? I was trying to grow the soft green algae- do you think I'll have to use tap water to grow the soft green algae since there's already diatoms in the tank, or can I grow both in the same bucket?